Saikawa reportedly admitted to having received, in 2013 when he was executive VP, nearly a half-million dollars in stock options he wasn't entitled to.

Saikawa, who had demanded Ghosn's immediate ouster when the executive was first arrested last November, said he will repay the funds, according to an official statement from Nissan.

Nissan CEO Hiroto Saikawa resigned Monday just days after the company's internal auditor who had led the Carlos Ghosn investigation resigned. Pictured at top, in happier days, is Saikawa with former chairman Ghosn during a shareholder meeting in June 2018.

In a statement, the Nissan board of directors said that Saikawa's last day will be September 16 and that chief operating officer Yasuhiro Yamauchi will be interim CEO until the end of October, when the company expects to find a replacement. Nissan said Saikawa had "indicated recently his willingness to resign." Another damning report from Nissan's internal investigation against Ghosn has also damned Saikawa, who was overpaid by more than $450,000 in stock options in 2013 while he was an executive vice president, the report said.

On Friday, Automotive News reported that Christina Murray, vice president of internal audit, had resigned "recently." She had been working with Nissan since the company began investigating Ghosn in October 2018 and was promoted to her current role in April. Nissan made no mention of Murray's resignation in its statements on Monday. When Tokyo police arrested Ghosn in November for financial improprieties, Saikawa—who was said to have been himself in Ghosn's crosshairs months earlier—reportedly demanded Nissan's board oust him immediately.

Saikawa had asked former representative director Greg Kelly—whom Nissan blames for approving and hiding Ghosn's allegedly illegitimate retirement payments—to increase his compensation that year. Kelly, according to Nissan, forged documents that showed Saikawa exercising his stock a week after he had already done so, which boosted his after-tax stock payment by 47 million yen (about $472,000 at the September 2013 exchange rate). Nissan said it found six more current or former directors and executive officers who received stock overpayments in this same fashion. Saikawa has indicated he will repay the amount, but Nissan will not charge him and others with misconduct "since they were unaware of the improper methods used by Kelly and others to increase their compensation."

Since the investigation began last October, Nissan alleges that Ghosn received more than $186 million in improper compensation—not all of which he was paid—and plucked another $140 million for personal and illegitimate expenses, according to the company's latest statement on the investigation. Ghosn's trial is yet to be scheduled.